Redox flow battery systems and methods utilizing a temporal energy profile

ABSTRACT

A redox flow battery system includes an anolyte; a catholyte; a first half-cell including a first electrode in contact with the anolyte; a second half-cell including a second electrode in contact with the catholyte; a separator separating the anolyte in the first half-cell from the catholyte in the second half-cell; at least one state measurement device configured for intermittently, periodically, or continuously making a measurement of a value indicative of a state of charge of the anolyte or the catholyte before entering or after leaving the first half-cell or second half-cell, respectively; and a controller coupled to the at least one state measurement device for generating a temporal energy profile of the anolyte or the catholyte, respectively, using the measurements.

RELATED PATENT APPLICATIONS

The present patent application claims the benefit of U.S. ProvisionalPatent Applications Ser. Nos. 63/120,204; 63/126,408; 63/127,048;63/131,738; 63/147,182; 63/154,547; 63/174,352; and 63/114,160, all ofwhich are incorporated herein by reference in their entireties.

FIELD

The present invention is directed to the area of redox flow batterysystems and methods of making and using redox flow battery systems. Thepresent invention is also directed to redox flow battery systems andmethods that utilize temporal energy profile for operation of the redoxflow battery system.

BACKGROUND

The cost of renewable power generation has reduced rapidly in the pastdecade and continues to decrease as more renewable power generationelements, such as solar panels, are deployed. However, renewable powersources, such as solar, hydroelectric, and wind sources, are oftenintermittent and the pattern of user load does not typically coincidewith the intermittent nature of the sources. There is a need for anaffordable and reliable energy storage system to store power generatedby renewable power sources when available and to provide power to userswhen there is insufficient power generation from the renewable powersources.

BRIEF SUMMARY

One embodiment is a redox flow battery system that includes an anolyte;a catholyte; a first half-cell including a first electrode in contactwith the anolyte; a second half-cell including a second electrode incontact with the catholyte; a separator separating the anolyte in thefirst half-cell from the catholyte in the second half-cell; at least onestate measurement device configured for intermittently, periodically, orcontinuously making a measurement of a value indicative of a state ofcharge of the anolyte or the catholyte before entering or after leavingthe first half-cell or second half-cell, respectively; and a controllercoupled to the at least one state measurement device for generating atemporal energy profile of the anolyte or the catholyte, respectively,using the measurements.

In at least some embodiments, the at least one state measurement deviceincludes an anolyte state measurement device configured forintermittently, periodically, or continuously making a measurement of avalue indicative of a state of charge of the anolyte before entering orafter leaving the first half-cell and a catholyte state measurementdevice configured for intermittently, periodically, or continuouslymaking a measurement of a value indicative of a state of charge of thecatholyte before entering or after leaving the second half-cell.

In at least some embodiments, the redox flow battery system furtherincludes an anolyte pump configured for pumping anolyte into and out ofthe first half-cell and a catholyte pump configured for pumpingcatholyte into or out of the second half-cell. In at least someembodiments, the controller is configured to use the temporal energyprofile to vary pumping speed of the anolyte and catholyte pumps.

In at least some embodiments, the at least one state measurement deviceis configured to measure a voltage of the anolyte or catholyte. In atleast some embodiments, the controller is configured to determine astate of charge of the anolyte or catholyte from the measurement made bythe at least one state measurement device and to use the state of chargein the temporal energy profile. In at least some embodiments, thecontroller is configured to determine an average oxidation state of theanolyte or catholyte from the measurement made by the at least one statemeasurement device and to use the average oxidation state in thetemporal energy profile. In at least some embodiments, the controller isconfigured to generate the temporal energy profile using themeasurements and an estimate of diffusion of charged species within theanolyte or catholyte.

Another embodiment is a method of operating any of the redox flowbattery systems described above. The method includes intermittently,periodically, or continuously making a measurement of a value indicativeof a state of charge of the anolyte or the catholyte before entering orafter leaving the first half-cell or second half-cell, respectively; andgenerating a temporal energy profile of the anolyte or the catholyte,respectively, using the measurements.

In at least some embodiments, the redox flow battery system furtherincludes an anolyte pump configured for pumping anolyte into and out ofthe first half-cell and a catholyte pump configured for pumpingcatholyte into or out of the second half-cell; the method furtherincluding using the temporal energy profile to vary pumping speed of theanolyte and catholyte pumps. In at least some embodiments, making ameasurement including making a measurement of a voltage of the anolyteor catholyte.

In at least some embodiments, generating the temporal energy profileincludes determining a state of charge of the anolyte or catholyte fromthe measurement made by the at least one state measurement device andusing the state of charge in the temporal energy profile. In at leastsome embodiments, generating the temporal energy profile includesdetermining an average oxidation state of the anolyte or catholyte fromthe measurement made by the at least one state measurement device andusing the average oxidation state in the temporal energy profile. In atleast some embodiments, generating the temporal energy profile includesgenerating the temporal energy profile using the measurements and anestimate of diffusion of charged species within the anolyte orcatholyte.

Yet another embodiment is a non-transitory computer-readable mediumhaving processor-executable instructions for operating any of the redoxflow battery systems described above. The processor-executableinstructions when installed onto a device enable the device to performactions. The actions include intermittently, periodically, orcontinuously making a measurement of a value indicative of a state ofcharge of the anolyte or the catholyte before entering or after leavingthe first half-cell or second half-cell, respectively; and generating atemporal energy profile of the anolyte or the catholyte, respectively,using the measurements.

In at least some embodiments, the actions further include using thetemporal energy profile to vary pumping speed of anolyte and catholytepumps of the redox flow battery system. In at least some embodiments,making a measurement including making a measurement of a voltage of theanolyte or catholyte.

In at least some embodiments, generating the temporal energy profileincludes determining a state of charge or average oxidation state of theanolyte or catholyte from the measurement made by the at least one statemeasurement device and using the state of charge or average oxidationstate in the temporal energy profile. In at least some embodiments,generating the temporal energy profile includes generating the temporalenergy profile using the measurements and an estimate of diffusion ofcharged species within the anolyte or catholyte.

A further embodiment is a redox flow battery system that includes ananolyte; a catholyte; a first half-cell including a first electrode incontact with the anolyte; a second half-cell including a secondelectrode in contact with the catholyte; a separator separating theanolyte in the first half-cell from the catholyte in the secondhalf-cell; at least one charging measurement device configured forintermittently, periodically, or continuously making a measurement ofpower supplied by a charging source; and a controller coupled to the atleast one charging measurement device for generating a temporal energyprofile of the anolyte or the catholyte, respectively, using themeasurements.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following drawings. In the drawings,like reference numerals refer to like parts throughout the variousfigures unless otherwise specified.

For a better understanding of the present invention, reference will bemade to the following Detailed Description, which is to be read inassociation with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of one embodiment of a redox flow batterysystem, according to the invention;

FIG. 2 is a schematic diagram of one embodiment of an electrode for aredox flow battery system, according to the invention;

FIG. 3 is a flowchart of a one embodiment of removing or reducingimpurities in a redox flow battery system, according to the invention;

FIG. 4 is a schematic diagram of another embodiment of a redox flowbattery system with the catholyte diverted into the second half-cell formaintenance, according to the invention;

FIG. 5A is a schematic diagram of one embodiment of a system thatincludes a redox flow battery system in conjunction with a balancingarrangement, according to the invention;

FIG. 5B is a schematic diagram of one embodiment of the balancingarrangement of the system of FIG. 5A, according to the invention;

FIG. 5C is a schematic diagram of another embodiment of a system thatincludes a redox flow battery system in conjunction with a balancingarrangement, according to the invention;

FIG. 5D is a schematic diagram of one embodiment of the balancingarrangement of the system of FIG. 5C, according to the invention;

FIG. 5E is a schematic diagram of another embodiment of a balancingarrangement, according to the invention;

FIG. 6A is a schematic diagram of electrolyte tanks of a redox flowbattery system with pressure release valves, according to the invention;

FIG. 6B is a schematic diagram of an electrolyte tank of a redox flowbattery system with a liquid-containing U-tube arrangement for pressurerelief, according to the invention;

FIG. 6C is a schematic diagram of electrolyte tanks of a redox flowbattery system with an arrangement for migration of gas between thetanks, according to the invention;

FIG. 7 is a schematic diagram of another embodiment of a redox flowbattery system with a secondary container, according to the invention;

FIG. 8 is a schematic diagram of another embodiment of a redox flowbattery system with a temperature zone, according to the invention;

FIG. 9A is a schematic cross-sectional view of one embodiment of anelectrode structure, according to the invention;

FIG. 9B is a schematic cross-sectional view of the electrode structureof FIG. 9A in a redox flow battery arrangement, according to theinvention;

FIG. 10 is a schematic diagram of one embodiment of a redox flow batterysystem with a primary redox flow battery arrangement and a secondaryredox flow battery arrangement, according to the invention;

FIG. 11 is a schematic diagram of one embodiment of a redox flow batterysystem coupled to an electrolysis cell for production of hydrogen gas,according to the invention;

FIG. 12 is a schematic diagram of one embodiment of a redox flow batterysystem with particulate filters, according to the invention;

FIG. 13 is a graph of charge capacity versus charge voltage for onebalanced redox flow battery system and two unbalanced redox flow batterysystems, according to the invention;

FIG. 14 is a flowchart of one embodiment of a method of determining anaverage oxidation state (AOS) of a redox flow battery system, accordingto the invention;

FIG. 15 is a flowchart of another embodiment of a method of determiningan average oxidation state (AOS) of a redox flow battery system,according to the invention;

FIG. 16 is a graph of self-discharge time versus discharge voltage foran unbalanced redox flow battery system, according to the invention;

FIG. 17 is a flowchart of a third embodiment of a method of determiningan average oxidation state (AOS) of a redox flow battery system,according to the invention;

FIG. 18 is a schematic diagram of yet another embodiment of a redox flowbattery system and includes a cell for measuring an open circuit voltage(OCV), according to the invention;

FIG. 19 is a flowchart of one method of determining storage or chargecapacity by measurement of an end OCV, according to the invention;

FIG. 20 is a flowchart of one method of determining AOS using ameasurement of OCV, according to the invention;

FIG. 21 is a schematic perspective illustration of multiple electrolytetanks disposed in a storage region; and

FIG. 22 is a schematic diagram of one embodiment of a redox flow batterysystem with state of charge measurement arrangements, according to theinvention; and

FIG. 23 is a flowchart of one embodiment of a method of operating aredox flow battery system using a temporal energy profile.

DETAILED DESCRIPTION

The present invention is directed to the area of redox flow batterysystems and methods of making and using redox flow battery systems. Thepresent invention is also directed to redox flow battery systems andmethods that utilize temporal energy profile for operation of the redoxflow battery system.

Redox flow battery systems are a promising technology for the storage ofenergy generated by renewable energy sources, such as solar, wind, andhydroelectric sources, as well as non-renewable and other energysources. As described herein, in at least some embodiments, a redox flowbattery system can have one or more of the following properties: longlife; reusable energy storage; or tunable power and storage capacity.

FIG. 1 illustrates one embodiment of a redox flow battery system 100. Itwill be recognized that other redox flow battery systems 100 may includemore or fewer elements and the elements may be arranged differently thanshown in the illustrated embodiments. It will also be recognized thatthe description below of components, methods, systems, and the like canbe adapted to other redox flow battery systems different from theillustrated embodiments.

The redox flow battery system 100 of FIG. 1 includes two electrodes 102,104 and associated half-cells 106, 108 that are separated by a separator110. The electrodes 102, 104 can be in contact or separated from theseparator. Electrolyte solutions flow through the half-cells 106, 108and are referred to as the anolyte 112 and the catholyte 114. The redoxflow battery system 100 further includes an anolyte tank 116, acatholyte tank 118, an anolyte pump 120, a catholyte pump 122, ananolyte distribution arrangement 124, and a catholyte distributionarrangement 126. The anolyte 112 is stored in the anolyte tank 116 andflows around the anolyte distribution arrangement 124 through, at leastin part, action of the anolyte pump 120 to the half-cell 106. Thecatholyte 114 is stored in the catholyte tank 118 and flows around thecatholyte distribution arrangement 126 through, at least in part, actionof the catholyte pump 122 to the half-cell 108. It will be recognizedthat, although the illustrated embodiment of FIG. 1 includes a singleone of each of the components, other embodiments can include more thanone of any one or more of the illustrated components. For example, otherembodiments can include multiple electrodes 102, multiple electrodes104, multiple anolyte tanks 116, multiple catholyte tanks 118, multiplehalf-cells 112, or multiple half-cells 114, or any combination thereof.

The anolyte and the catholyte are electrolytes and can be the sameelectrolyte or can be different electrolytes. During energy flow into orout of the redox flow battery system 100, the electrolyte in one of thehalf-cells 106, 108 is oxidized and loses electrons and the electrolytein the other one of the half-cells is reduced and gains electrons.

The redox flow battery system 100 can be attached to a load/source130/132, as illustrated in FIG. 1. In a charge mode, the redox flowbattery system 100 can be charged or recharged by attaching the flowbattery to a source 132. The source 132 can be any power sourceincluding, but not limited to, fossil fuel power sources, nuclear powersources, other batteries or cells, and renewable power sources, such aswind, solar, or hydroelectric power sources. In a discharge mode, theredox flow battery system 100 can provide energy to a load 130. In thecharge mode, the redox flow battery system 100 converts electricalenergy from the source 132 into chemical potential energy. In thedischarge mode, the redox flow battery system 100 converts the chemicalpotential energy back into electrical energy that is provided to theload 130.

The redox flow battery system 100 can also be coupled to a controller128 that can control operation of the redox flow battery system. Forexample, the controller 128 may connect or disconnect the redox flowbattery system 100 from the load 130 or source 132. The controller 128may control operation of the anolyte pump 120 and catholyte pump 122.The controller 128 may control operation of valves associated with theanolyte tank 116, catholyte tank 118, anolyte distribution system 124,catholyte distribution system 126, or half-cells 106, 108. Thecontroller 128 may be used to control general operation of the redoxflow battery system 100 include switching between charge mode, dischargemode, and, optionally, a maintenance mode (or any other suitable modesof system operation.) In at least some embodiments, the controller orthe redox flow battery system may control the temperature of within thehalf-cells or elsewhere in the system. In at least some embodiments, thetemperature of the half-cells (or the system in general or portions ofthe system) is controlled to be no more than 65, 60, 55, or 50 degreesCelsius during operation.

In at least some embodiments, the anolyte pump 120 or catholyte pump 122(or both) can be operated to increase or maintain the temperature of theanolyte/catholyte or the half-cells. Operation of the pumps generatesheat that can be transferred, at least in part, to the anolyte orcatholyte. In at least some embodiments, if the temperature of theanolyte or catholyte (or the corresponding half-cell 106, 108) fallsbelow a pre-determined value the anolyte pump 120 or catholyte pump 122,respectively, initiates or increases operation to generate heat that istransferred, at least in part, to the anolyte or catholyte,respectively.

Any suitable controller 128 can be used including, but not limited to,one or more computers, laptop computers, servers, any other computingdevices, or the like or any combination thereof and may includecomponents such as one or more processors, one or more memories, one ormore input devices, one or more display devices, and the like. Thecontroller 128 may be coupled to the redox flow battery system throughany wired or wireless connection or any combination thereof. Thecontroller 128 (or at least a portion of the controller) may be locatedlocal to the redox flow battery system 100 or located, partially orfully, non-locally with respect to the redox flow battery system.

The electrodes 102, 104 can be made of any suitable material including,but not limited to, graphite or other carbon materials (including solid,felt, paper, or cloth electrodes made of graphite or carbon), gold,titanium, lead, or the like. The two electrodes 102, 104 can be made ofthe same or different materials. In at least some embodiments, the redoxflow battery system 100 does not include any homogenous or metalliccatalysts for the redox reaction in the anolyte or catholyte or both.This may limit the type of material that may be used for the electrodes.

The separator 110 separates the two half-cells 106, 108. In at leastsome embodiments, the separator 110 allows the transport of selectedions (for example, Cl⁻, or iron or chromium ions or any combinationthereof) during the charging or discharging of the redox flow batterysystem 100. In some embodiments, the separator 110 is a microporousmembrane. Any suitable separator 110 can be used and examples ofsuitable separator include, but are not limited to, ion transfermembranes, anionic transfer membranes, cationic transfer membranes,microporous separators, or the like or any combination thereof.

An alternative to the electrodes 102, 104 and separator 110 is anelectrode structure (which can be referred to as a “bipolar electrode”)905 that acts as an anode, a cathode, and a separator, as illustrated inFIG. 9A. The electrode structure 905 includes a base 910 with conductiveelements 911 that extend through the base from a first surface of thebase to a second surface of the base. These conductive elements 911 areexposed in the two half-cells 906, 908, as illustrated in FIG. 9B. Thebase 910 can act as the separator 110.

The base 910 is made of a material that is impermeable to, orsubstantially resists or hinders flow of, the anolyte and catholytethrough the base. Examples of such materials include plastic (such aspolypropylene, polyethylene, perfluoroalkoxy alkane (PFA),polyvinylidene fluoride (PVDF), polytetrafluoroacetic acid (PTFE),polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), or thelike), a resin, a carbon or graphite plate, a metal plate, a metal-alloyplate, or the like. In at least some embodiments, the base isnon-conductive. In at least some embodiments, the base is conductive,such as a graphite plate. In at least some embodiments, the conductivityof the base 910 between the first surface and the second surface is lessthan the conductivity of the conductive elements 911.

The conductive elements 911 extend through the base 910 so that they areaccessible to both the anolyte and catholyte. Examples of suitableconductive elements include, but are not limited to, metal wires (forexample, titanium iron, copper, zinc, silver, gold, or platinum wires),carbon fibers, graphite fibers aligned for electron conduction throughthe base, silicon carbide, or the like or any combination thereof. Theconductive elements 911 form part of (or the entirety of) electrodes902, 904 which are disposed on the opposing first and second surfaces ofthe base 910. Optionally, electrodes 902, 904 can include additionalconductive material, such as metal, carbon fiber, carbon felt, siliconcarbide, or the like, that does not extend through the base 910 but isdisposed on the surface of the base or extend into, but not through, thebase.

In at least some embodiments, the base 910 can be molded with theconductive elements 911 arranged so that the conductive elements willextend through the base. In at least some embodiments, the conductiveelements 911 may be inserted or pushed through the base 910. In at leastsome embodiments, the base 910 can be heated after passing theconductive elements 911 through the base to flow the material of thebase and embed medial portions of the conductive elements within thebase.

Although the electrode structure 905 is disclosed herein in the contextof a Fe—Cr redox flow battery system, it will be recognized that theelectrode structure 905 can be used in other redox flow battery systemsincluding, but not limited to, vanadium redox flow battery systems,vanadium-bromine redox flow battery systems, vanadium-iron redox flowbattery systems, zinc-bromine redox flow battery systems, all iron redoxflow battery systems, organic aqueous redox flow battery systems, or thelike. The electrode structure 905 can also be used in otherelectrochemical systems and methods.

Redox flow battery systems can be safe, reliable, and provide a reusableenergy storage medium. It has been challenging, however, to identify aredox flow battery system that has a desirable storage energy with along life (e.g., a flow battery system that maintains its storagecapacity for many charge/discharge cycles) and is made of materials thathave abundant availability (e.g., materials that are abundant on Earthand are commercially mined and available in relatively largequantities). Current lithium and vanadium batteries utilize materialsthat have limited availability. The storage capacity of manyconventional battery systems also degrades when subjected 10, 50, or 100charge/discharge cycles or more. A further challenge for aqueous redoxflow battery systems is to manage or avoid the evolution of hydrogen oroxygen from water.

As described herein, a suitable and useful redox flow battery system isan iron-chromium (Fe—Cr) redox flow battery system utilizing Fe³⁺/Fe²⁺and Cr³⁺/Cr²⁺ redox chemistry. Iron and chromium are generally readilycommercially available and, at least in some embodiments, the storagecapacity of a Fe—Cr redox flow battery system does not degrade by morethan 10% or 20% over at least 100, 200, 250, or 500 charge/dischargecycles or can be configured, using maintenance procedures, to maintainat least 70%, 80%, or 90% storage capacity over at least 100, 200, 250,or 500 charge/discharge cycles.

In at least some embodiments, the electrolytes (i.e., the catholyte oranolyte) of a Fe—Cr redox flow battery system include an iron-containingcompound or a chromium-containing compound (or both) dissolved in asolvent. In some embodiments, the anolyte and catholyte contain both theiron-containing compound and the chromium-containing compound. Theconcentrations of these two compounds in the anolyte and catholyte canbe the same or different. In other embodiments, the catholyte includesonly the iron-containing compound and the anolyte includes only thechromium-containing compound.

In at least some embodiments, the chromium-containing compound can be,for example, chromium chloride, chromium sulfate, chromium bromide, orthe like or any combination thereof. In at least some instances, it hasbeen found that chloride-complexed chromium ions (for example,Cr(H₂O)₅Cl^(2+/+)) have faster reaction kinetics and lower H₂ productionthan at least some other chromium ion complexes (for example, Cr(H₂O)₆^(3+/2+)). Accordingly, the inclusion of chloride in the anolyte (forexample, from the chromium-containing compound, the solvent, or both)can be beneficial.

It has been found that chromium can form complexes withnitrogen-containing ligands that, at least in some instances, are morestable than chloride complexes of chromium. In at least some instances,the chromium complexes with nitrogen-containing ligands may be moreredox active or may result in fewer side reactions (such as hydrogengeneration) in the Fe—Cr redox flow battery. In at least someembodiments, the chromium-containing compound can be a chromium complexincluding at least one of the following nitrogen-containing ligands:ammonia (NH₃), ammonium (NH₄ ⁺), urea (CO(NH₂)₂), thiocyanate (SCN⁻), orthiourea (CS(NH₂)₂) or any combination thereof. Examples of complexeswith combinations of different nitrogen-containing ligands includechromium complexes with ammonia and urea.

In at least some embodiments, the chromium complex has the formula[Cr³⁺(J)_(x)(M)_(y)(H₂O)_(z)] where x is a positive integer and y and zare non-negative integers with x+y+z=6, J is selected from the groupconsisting of NH₃, NH₄ ⁺, CO(NH₂)₂, SCN⁻, or CS(NH₂)₂, and each M isdifferent from J and independently selected from the group consisting ofCl⁻, F⁻, Br⁻, I⁻, NH₄ ⁺, NH₃, ethylenediaminetetraacetic acid (EDTA),CN⁻, SCN⁻, S²⁻, O—NO₂ ⁻, OH⁻, NO₂ ⁻, CH₃CN, C₅H₅N, NC₅H₄—C₅H₄N, C₁₂H₈N₂,CO(NH₂)₂, CS(NH₂)₂, P(C₆H₅)₃, —CO, CH₃—CO—CH₂—CO—CH₃, NH₂—CH₂—CH₂—NH₂,NH₂CH₂COO⁻, O—SO₂ ²⁻, or P(o-tolyl)₃. This chromium-containing compoundcan also include any suitable counterions including, but not limited to,ammonium, chloride, bromide, iodide, fluoride, sulfate, nitrate, or thelike or any combination thereof. One example of a chromium complex isCr(NH₃)_(x)Cl_(y)(H₂O)_(z) where x and z are in the range of 1 to 6 andy is in the range of 1 to 3. In at least some embodiments, the ratio ofCr to NH₃ (or other nitrogen-containing ligand) for the anolyte orcatholyte can be less than 1 as long as a portion of the chromium ionsare complexed with ammonia (or other nitrogen-containing ligand).

In at least some embodiments, the chromium complex can be createdin-situ in the electrolyte (either the anolyte or catholyte or both) byexposing a chromium salt (for example, chromium chloride, chromiumsulfate, chromium bromide, or the like or any combination thereof) orother chromium compound to a ligand-containing compound (for example,ammonia, ammonium chloride, urea, potassium thiocyanate, sodiumthiocyanate, or thiourea.)

In at least some embodiments, the molar ratio of the nitrogen-containingligand(s) to chromium in the electrolyte (either the anolyte orcatholyte or both) is in the range of 1:10 to 10:1. In the case of achromium complex with ammonia and urea, the molar ratio of ammonia tourea is in the range of 1:10 to 10:1. It will be understood that atleast some of the nitrogen-containing ligand(s) may not be complexedwith chromium. For example, at least some of the nitrogen-containingligand(s) may be complexed with iron or may not be complexed within theelectrolyte.

The iron-containing compound can be, for example, iron chloride; ironsulfate; iron bromide; an iron complex including at least one of ammonia(NH₃), ammonium (NH₄ ⁺), urea (CO(NH₂)₂), thiocyanate (SCN⁻), orthiourea (CS(NH₂)₂) as a ligand; or the like or any combination thereof.In at least some embodiments, the iron complex can include the sameligands as a chromium complex in the same electrolyte (either theanolyte or catholyte or both) or a subset of those ligands.

The solvent can be water; an aqueous acid, such as, hydrochloric acid,hydrobromic acid, sulfuric acid, or the like; or an aqueous solutionincluding a soluble salt of a weak acid or base, such as ammoniumchloride. In at least some embodiments, the water content of the anolyteor catholyte (or both) is at least 40, 45, or 50 wt. %. In at least someembodiments, both the catholyte and the anolyte of an Fe—Cr redox flowbattery system includes iron chloride and chromium chloride dissolved inhydrochloric acid. In at least some embodiments, the catholyte of anFe—Cr redox flow battery system includes iron chloride dissolved inhydrochloric acid and the anolyte includes chromium chloride dissolvedin hydrochloric acid.

In at least some embodiments, a nitrogen-containing compound may alsoprovide benefits relative to the solvent even if the nitrogen-containingcompound is not a ligand of chromium or iron. For example, urea orthiourea in the electrolyte (either the anolyte or catholyte or both)can neutralize HCl in the electrolyte which may reduce HCl vapor duringbattery operation. As another example, a solvent with ammonium orammonium ions (for example, replacing all or part of the hydrochloricacid with ammonium chloride) can result in an effective electrolyte(either the anolyte or catholyte or both) that has lower acidity.

In at least some embodiments, both the catholyte and the anolyte of anFe—Cr redox flow battery system includes iron chloride and chromiumchloride dissolved in hydrochloric acid. In at least some embodiments,the catholyte of an Fe—Cr redox flow battery system includes ironchloride dissolved in hydrochloric acid and the anolyte includeschromium chloride dissolved in hydrochloric acid.

In at least some embodiments, the molarity of iron in the catholyte orthe anolyte or both is in a range of 0.5 to 2 or is at least 1 M. In atleast some embodiments, the molarity of chromium in the anolyte or thecatholyte or both is in a range of 0.1 to 2 or is at least 0.2, 0.5, or1 M. In at least some embodiments, the molarity of the hydrochloric acidor other aqueous acid or base is in a range of 0.5 to 2. In at leastsome embodiments, the molarity of ammonia or ammonium ions is in a rangeof 0.5 to 4.

As one example of a method for forming an anolyte or catholyte withchromium complex, a 100 gram mixture of FeCl₂.4H₂O (35 wt. %),CrCl₃.6H₂O (45 wt. %), and NH₄Cl (20%) was added to a 250 mL beaker anddissolved in distilled ionized water to form a 150 mL solution. Stirringof the solution at 50° C. to 60° C. accelerated dissolving. 2 mL of 37wt. % HCl was added to the solution to adjust the pH.

In another example, a 100 gram mixture of FeCl₂.4H₂O (32 wt. %),CrCl₃.6H₂O (43 wt. %), and CO(NH₂)₂ (25%) was added to a 250 mL beakerand dissolved in distilled ionized water to form a 150 mL solution. 1.5mL of 37 wt. % HCl was added to the solution to adjust the pH.

As a third example, a 100 gram mixture of FeCl₂.4H₂O (35 wt. %),CrCl₃.6H₂O (45 wt. %), NH₄Cl (10%), and CO(NH₂)₂ (10%) was added to a250 mL beaker and dissolved in distilled ionized water to form a 150 mLsolution. 2 mL of 37 wt. % HCl was added to the solution to adjust thepH.

One challenge of previous Fe—Cr redox flow batteries is the generationor evolution of hydrogen (H₂) at the negative electrode as a result ofthe redox reactions. In at least some instances, increasing theutilization of the chromium in the redox flow battery can increase theproduction of hydrogen.

It is often desirable to limit or reduce the production of hydrogen inthe redox flow battery. It has been found that limiting the utilizationof chromium results in lower hydrogen generation while retainingadequate energy density in the redox flow battery system. In at leastsome embodiments, the chromium utilization in the anolyte of the redoxflow battery system is limited to no more than 80%, 70%, or 60% or less.In at least some embodiments, the chromium utilization in the anolyte islimited by amount of iron in the catholyte or is limited by 100%utilization of the iron in the catholyte.

Chromium utilization can be managed, at least in part, by managing therelative amounts of chromium and iron in the redox flow battery system.The term “molar ratio” as used herein means the ratio of the molaramount of one component with respect to the molar amount of a secondcomponent. In at least some embodiments, the molar ratio of chromium inthe anolyte to iron in the catholyte (Cr(anolyte)/Fe(catholyte)) is not1, but, instead, the Cr(anolyte)/Fe(catholyte) molar ratio is at least1.25 or more (for example, at least 1.43, 1.67, or more). In at leastsome embodiments, the molar amount of iron in the catholyte is no morethan 80%, 70%, or 60% or less of the molar amount of chromium in theanolyte. In at least some embodiments, the smaller amount of availableiron limits the utilization of the available chromium to no more than80%, 70%, or 60%. In at least some embodiments, the anolyte and thecatholyte are both mixed iron/chromium solutions.

In at least some embodiments, the concentration of iron in the catholyteis different from the concentration of chromium in the anolyte toproduce the desired molar ratio. In at least some embodiments, theconcentration of iron in the catholyte is no more than 80%, 70%, or 60%or less of the concentration of chromium in the anolyte.

In at least some embodiments, the concentration of iron in the catholyteand the concentration of chromium in the anolyte is the same. In suchembodiments, the molar ratio of chromium and iron in the anolyte andcatholyte, respectively, can be selected by selection of the volumes ofthe anolyte and catholyte. In at least some embodiments, the volumeratio of anolyte to catholyte is at least 1.25:1 or more (for example,at least 1.43:1 or 1.67:1 or more) leading to a molar ratio that isequal to the volume ratio when the concentrations of chromium in theanolyte and iron in the catholyte are the same. In at least someembodiments, the volume of the catholyte is no more than 80%, 70%, or60% of the volume of the anolyte.

In some embodiments, the volumes of the anolyte and the catholyte can bebased on the volume of the respective half-cells 106, 108. In someembodiments, the volumes of the anolyte and the catholyte can be basedon the volume of the respective catholyte and anolyte portions of theredox flow battery system 100. For example, the catholyte portion caninclude the half-cell 108, the catholyte tank 118, and the catholytedistribution arrangement 126. The anolyte portion can include thehalf-cell 106, the anolyte tank 116, and the anolyte distributionarrangement 124.

It will be recognized that a combination of both different iron andchromium concentrations and different catholyte and anolyte volumes canbe used to achieve the desired molar ratio of chromium in the anolyteand iron in the catholyte. In at least some of these embodiments, thevolume of the catholyte is no more than 95%, 90%, 80%, 70%, or 60% ofthe volume of the anolyte.

In at least some instances, it is found that higher H⁺ concentration inthe anolyte promotes hydrogen generation. To reduce hydrogen generationby the anolyte, the H⁺ concentration in the initial anolyte can be lowerthan the H⁺ concentration in the initial catholyte. In at least someembodiments, the H⁺ concentration in the initial anolyte is at least 10,20, 25, or 50 percent lower than the H⁺ concentration in the initialcatholyte.

Table 1 illustrates a 1:1 volume ratio of anolyte to catholyte atdifferent states of charge (SOC) where the state of charge representsthe percentage conversion of the initial active ionic species in theanolyte and catholyte to the reduced/oxidized ionic species. It will berecognized that the concentration of H⁺ changes to maintain chargebalance between the anolyte and catholyte. In Table 1, the initialanolyte is 1.25M Fe²⁺, 1.25M Cr³⁺, and 1.25M H⁺ and the initialcatholyte is 1.25M Fe²⁺, 1.25M Cr³⁺, and 2.5M H⁺. These particularconcentrations are selected so that the H⁺ concentration is equal at the50% state of charge.

TABLE 1 State of Anolyte Catholyte Charge Cr²⁺ Cr³⁺ H⁺ Fe²⁺ Fe³⁺ H⁺ 0 01.25 1.25 1.25 0 2.5 25 0.3125 0.9375 1.5625 0.9375 0.3124 2.1875 500.625 0.625 1.875 0.625 0.625 1.875 75 0.9375 0.3125 2.1875 0.31250.9375 1.5625 100 1.25 0 2.5 0 1.25 1.25

Table 2 illustrates a 2:1 volume ratio of anolyte to catholyte atdifferent states of charge (SOC). In Table 2, the initial anolyte is1.25M Fe²⁺, 1.25M Cr³⁺, and 1.5625M H⁺ and the initial catholyte is1.25M Fe²⁺, 1.25M Cr³⁺, and 2.5M H⁺. These particular concentrations areselected so that the H⁺ concentration is equal when the anolyte is at25% SOC and the catholyte is at 50% SOC. The difference in SOC betweenthe anolyte and catholyte arises due to anolyte having twice the volumeof the catholyte.

TABLE 2 State of Anolyte Catholyte Charge Cr²⁺ Cr³⁺ H⁺ Fe²⁺ Fe³⁺ H⁺ 0 01.25 1.5625 1.25 0 2.5 25 0.3125 0.9375 1.875 0.9375 0.3124 2.1875 500.625 0.625 2.1875 0.625 0.625 1.875 75 0.3125 0.9375 1.5625 100 0 1.251.25

Another challenge with Fe—Cr redox flow battery systems, as well asother redox flow battery systems, is the presence of metal impurities,such as nickel, antimony, zinc, bismuth, platinum, gold, or copper. Inat least some instances, these metal impurities can increase hydrogengeneration on the negative electrode surface. Such metallic impuritiescan be present as a natural impurity or as a part of the refining ormanufacturing of the iron and chromium compounds or other portions ofthe redox flow battery system or through any other mechanism.

In at least some embodiments, the redox flow battery system 100 can beconfigured to remove, or reduce the level of, these impurities. Asillustrated in FIG. 3, in at least some embodiments, to remove, orreduce the level of, these impurities, the redox flow battery system 100is configured to electrochemically reduce at least some of theimpurities to metal form (step 350), collect the resulting metallicparticles using a particulate filter or other arrangement such at theinterdigitated electrode described below (step 352), and remove theseimpurities using a cleaning solution containing an oxidizing species(step 354).

In at least some embodiments, the impurities are reduced within theanolyte as part of the redox reactions. The impurities form metallicparticles or particulates when reduced during charging. The redox flowbattery system 100 may include a particulate filter in the half-cell 106or elsewhere to capture the metallic particles or particulates. In someembodiments, the negative electrode 102 may aid in filtering themetallic particles or particulates. To also facilitate the removal ofthe impurities, the negative electrode 102 can have an interdigitatedstructure, as illustrated in FIG. 2. The interdigitated structureincludes empty or indented channels 240 for collection of particles ofthe metallic impurities during operation of the redox flow batterysystem 100. These particles can then be removed from the electrodeduring a maintenance cycle, as described below. FIG. 12 illustratesexamples of alternative sites for a particulate filter 121 including,but not limited to, upstream or downstream of the anolyte/catholytetanks 116, 118; downstream or upstream of the half-cells 106, 108, orthe like or any combination thereof. In at least some embodiments, thefilter 121 has a pore size in a range of 1-10 micrometers. In at leastsome embodiments, the positive and negative assignment of the electrodes102, 104 can be reversed so that the impurities in both electrolytetanks can be removed.

In at least some embodiments, the Fe—Cr redox flow battery systemsdescribed herein are arranged to remove these impurities using asolution with an oxidizing species, such as Fe³⁺. As part of themaintenance of the redox flow battery system 100, during a maintenancecycle, a Fe³⁺ (or other oxidizing) solution can be flowed through theanolyte portion of the system to remove the impurities from theelectrode 102 or elsewhere in the system. In at least some embodiments,the Fe³⁺ solution can be the catholyte or a portion of the catholyte.Alternative oxidizing solutions include, but are not limited to,hydrogen peroxide solutions, ferric chloride solutions, nitric acid, orthe like. In at least some embodiments, the redox flow battery system100 can include a cleaning solution tank (not shown) than can be coupledto the anolyte/catholyte distribution arrangements 124, 126 (orotherwise coupled to the filters 121) to periodically clean the filters.

In at least some embodiments, the removal or reduction of metallicimpurities is performed during manufacturing of the redox flow batterysystem, prior to the onset of operation of the redox flow batterysystem, or during operation of the redox flow battery system, or anycombination thereof. It will be understood that these methods andsystems for removal of metallic impurities are not limited to Fe—Crredox flow battery systems but can also be utilized in other redox flowbattery systems such as vanadium, vanadium-bromine, vanadium-iron,zinc-bromine, and organic redox flow battery systems.

It has also been found that, in at least some embodiments, occasionalexposure of the electrode 102 to the catholyte 114 can facilitatepassivation of the surface of the electrode 102 and reduce hydrogengeneration. As an example, in one Fe—Cr redox flow battery system theelectrode 102 was treated with the catholyte 114 for 1 hour after 17charge/discharge cycles and the hydrogen generation rate when down from38.9 ml/min to 10.2 ml/min. In at least some embodiments, operation ofthe redox flow battery system can periodically (or when initiated orrequested by an operator) include a maintenance period in which thehalf-cell 106 or electrode 102 is exposed to the catholyte (or anelectrolyte that has components such as those specified above for thecatholyte) for a period of time (for example, 5, 10, 15, 30, 45, 60minutes or more.) The catholyte may be introduced to the half-cell 106or electrode 102 once, periodically, intermittently, or continuouslyduring the maintenance period. In at least some of these embodiments,the catholyte 114 can be returned to the catholyte tank 118 after themaintenance period. In at least some embodiments, the maintenance periodmay be performed when the state of charge of the anolyte is at least50%, 75% or 90%.

FIG. 4 illustrates one embodiment of a redox flow battery system thatincludes switches 434 for disconnecting the anolyte distribution system124 from the half cell 106 and connecting the catholyte distributionsystem 126 to the half-cell 106 to flow catholyte 114 into the half-cell106. Such an arrangement can be used to reduce or remove metallicimpurities or to passivate the electrode 102 or any combination thereof.The pump 122 can be used to flow catholyte 114 into the half-cell 106 orto remove the catholyte 114 from the half-cell 106 when the maintenanceis complete.

A Fe—Cr redox flow battery system can have a reduction in storagecapacity over time arising, at least in part, from the low standardpotential of the Cr²⁺/Cr³⁺ pair which results in at least some level ofhydrogen generation on the anolyte side of the system. As a result, theAverage Oxidation State (AOS) of the active species in the systemincreases and the system can become unbalanced and the storage capacitydecrease. It is useful, therefore, to have methods or arrangements forat least partially restoring the storage capacity by recovering the AOS.

In at least some embodiments, the AOS for a Fe—Cr redox flow batterysystem can be described as: AOS=((Moles of Fe³⁺ in catholyte andanolyte)*3+(Moles of Fe²⁺ in catholyte and anolyte)*2+(Moles of Cr³⁺ inanolyte and catholyte)*3+(Moles of Cr²⁺ in anolyte andcatholyte)*2)/(Moles of Fe in catholyte and anolyte+Moles of Cr inanolyte and catholyte).

In at least some embodiments, the presence of ammonia or urea in theelectrolytes (for example, as ligands of the chromium complex) canfacilitate rebalancing of the system and restoration of the storagecapacity. In at least some embodiments, the following electrolyticreactions occur at the electrodes:

7H₂O+2Cr³⁺−6e⁻→Cr₂O₇ ²⁻+14H⁺E₀=+1.33 V

Fe³⁺+e⁻→Fe²⁺ E₀=+0.77 V

The chromate ions can react with urea or ammonia to regenerate Cr³⁺ torebalance the system:

Cr₂O₇ ²⁻+8H⁺+CO(NH₂)₂→2Cr³⁺+CO₂+N₂+6H₂O

Cr₂O₇ ²⁻+8H⁺+2NH₃→2Cr³⁺+N₂+7H₂O

In at least some embodiments, the resulting nitrogen or carbon dioxidecan be released to prevent pressurization of the redox flow batterysystem.

Alternatively or additionally, in at least some embodiments, torebalance the redox flow battery system the redox flow battery systemincludes a balance arrangement, in conjunction with either the anolyteor catholyte, to rebalance the system and restore storage capacity. Inat least some embodiments, the balance arrangement utilizes a vanadiumsource (to produce oxovanadium (VO²⁺) and dioxovanadium (VO₂ ⁺) ionicspecies) and a reductant, such as an oxidizable hydrocarbon compound, torebalance the system and restore storage capacity. The followingembodiments illustrate the addition of a balance arrangement to a Fe—Crredox flow battery system. It will be understood that such balancearrangements can be used with other redox flow battery systems, or otherchemical and/or electrochemical systems.

FIG. 5A illustrates one embodiment of portions of the redox flow batterysystem 100 and a balance arrangement 500. FIG. 5B illustrates oneembodiment of the balance arrangement 500. In this embodiment, thecatholyte 114 is used in conjunction with a balancing electrolyte 562(for example, an electrolyte containing VO²⁺/VO₂ ⁺) and a reductant 563to rebalance the redox flow battery system 100. The balance arrangement500 includes the catholyte tank 118; balance electrodes 552, 554;balance half-cells 556, 558; balance separator 560; catholyte balancepump 572; catholyte balance distribution system 576; balance tank 566;optional reductant tank 567; balance electrolyte pump 570; balanceelectrolyte distribution arrangement 574; and potential source 561. Inat least some embodiments, the reductant can be urea or ammonia whichmay be present as ligands of a chromium or iron complex or can beotherwise provided as a reductant.

The following reaction equations illustrate one example of therebalancing of the system using the iron-based catholyte 114, abalancing electrolyte 562 containing oxovanadium ions, and a reductant563 containing urea or ammonia.

VO²⁺+H₂O+Fe³⁺→VO₂ ⁺+Fe²⁺+2H⁺

6VO₂ ⁺+6H⁺+CO(NH₂)₂→6VO²⁺+CO₂+N₂+5H₂O

6VO₂ ⁺+6H++2NH₃→6VO²⁺+N₂+6H₂O

In at least some embodiments, the resulting nitrogen or carbon dioxidecan be released to prevent pressurization of the redox flow batterysystem.

The following reaction equations illustrate another example of therebalancing of the system using the iron-based catholyte 114, abalancing electrolyte 562 containing oxovanadium ions, and a reductant563 containing fructose, along with the application of an externalpotential from the potential source 561 of at least 0.23 V:

VO²⁺+H₂O+Fe³⁺→VO₂ ⁺+Fe²⁺+2H⁺

24VO₂₊+24H⁺+C₆H₁₂O₆→24VO²⁺+6CO₂+18H₂O

Via the reactions illustrated in the two examples above, the AOS of theredox flow battery system 100 can be reduced and the H⁺ ions lost inhydrogen generation restored. In at least some embodiments, thisrebalancing (or restoring of the AOS or storage capacity recovery) doesnot utilize any metallic catalyst as such catalysts often increasehydrogen generation. In at least some embodiments, VO²⁺ of the balanceelectrolyte 562 can be considered a homogeneous catalyst as the VO²⁺ions are regenerated using the reductant 563. In at least someembodiments, the reduction of VO₂ ⁺ ions happens in balance half cell566.

In at least some embodiments, the oxidation of the reductant 563 can beperformed in the balance tank 566 instead of the half-cell 556 and maynot require the application of an external potential, as long as VO₂ ⁺ions are available. Suitable reducing agents include sugars (forexample, fructose, glucose, sucrose, or the like or any combinationthereof), carboxylic acids (for example, formic acid, acetic acid,propionic acid, oxalic acid, or the like or any combination thereof),aldehydes (for example, formaldehyde, acetaldehyde, or the like or anycombination thereof), alcohols (for example, methanol, ethanol,propanol, or the like or any combination thereof), ammonia, urea,thiourea, ammonium ions, other hydrocarbons, or hydrogen gas. In atleast some embodiments, the reductant is soluble or at least partiallysoluble in water.

In at least some embodiments, the reductant 563 is added eitherperiodically, intermittently, or continuously to the balance electrolyte562 from the reductant tank 567. In at least some embodiments, thisrebalancing process (for recovering the storage capacity or restoringthe AOC) occurs continuously, intermittently, or periodically. Forexample, the catholyte balance pump 572 and balance electrolyte pump 570can operate continuously, intermittently, or periodically. In at leastsome embodiments, the catholyte pump 122 can also be used as thecatholyte balance pump 572. Moreover, the catholyte balance distributionarrangement 576 may include a valve to couple to, or disconnect from,the catholyte tank 118.

FIGS. 5C and 5D illustrate another embodiment of redox flow batterysystem 100 with a balance arrangement 500′ which operates with theanolyte 112 (and corresponding anolyte pump 572′ and anolyte balancedistribution arrangement 576′) instead of the catholyte. In at leastsome embodiments, the anolyte pump 120 can also be used as the anolytebalance pump 572′.

The following reaction equations illustrate one example of therebalancing of the system using the chromium-based anolyte 112, abalancing electrolyte 562 containing oxovanadium ions, and a reductant563 containing fructose, along with the application of an externalpotential from the potential source 561 of at least 1.40 V:

VO²⁺+H₂O+Cr³⁺→VO₂ ⁺+Cr²⁺+2H⁺

24VO₂ ⁺+24H⁺+C₆H₁₂O₆→24VO²⁺+6CO₂+18H₂O

Other reductants, including those listed above, can be used instead offructose.

FIG. 5E illustrates another embodiment of a balance arrangement 500″which can be adapted to operate with either the catholyte or anolyte andthe corresponding catholyte/anolyte tank 118/116 that is coupled to theremainder of the redox flow battery system 100. This embodimentincorporates an intermediate tank 584 and two intermediate half-cells586, 588 between the catholyte/anolyte tank 118/116 and the balance tank562 and corresponding half-cells 556/558. (As with the balance tank,there can be an intermediate pump and intermediate distributionarrangement, as well as an intermediate separator between the twohalf-cells 586, 588 and a source potential to apply a potential betweenthe electrodes of the two half-cells 586, 588.) In one embodiment, theintermediate electrolyte in the intermediate tank 584 contains V²⁺/V³⁺ions.

The following reaction equations illustrate one example of therebalancing of the system using balance arrangement 500″ and thecatholyte 114 of redox flow battery system 100 (FIG. 1).

VO²⁺+H₂O−e⁻→VO₂ ⁺+2H⁺ (half-cell 556)

V³⁺+e⁻→V²⁺ (half-cell 558)

V²⁺−e⁻→V³⁺ (half-cell 586)

Fe³⁺+e⁻→Fe²⁺ (half-cell 588)

24VO₂ ⁺+24H⁺+C₆H₁₂O₆→24VO²⁺+6CO₂+18H₂O (balance tank 562 or half cell556 or both)

Other reductants, including those listed above, can be used instead offructose.

Another embodiment uses the anolyte (Cr²⁺/Cr³⁺) instead of the catholytein conjunction with the intermediate electrolyte and balanceelectrolyte. Yet another embodiment uses the anolyte and replaces theV²⁺/V³⁺ intermediate electrolyte with a Fe²⁺/Fe³⁺ intermediateelectrolyte.

It will be recognized that the balance arrangement described herein canbe utilized with other redox flow battery systems and, in particular,those that are capable of generating hydrogen gas. Examples of suchredox flow battery system include, but are not limited to, Zn—Br orZn—Cl redox flow battery systems, vanadium-based (for example, allvanadium, V—Br, V—Cl, or V-polyhalide) redox flow battery systems; Fe—Vor other iron-based redox flow battery systems (for example, an all ironredox flow battery system); or organic redox flow battery systems.

In some embodiments, during Fe²⁺-overcharging conditions, chlorine gas(Cl₂) can be generated on the catholyte side of the redox flow batterysystem 100. The chlorine may be confined in the catholyte headspace of,for example, the catholyte tank 118 or half-cell 108 or the like or anycombination thereof. Continued generation of chlorine gas increases thepressure in the confined catholyte headspace. In at least someembodiments, this may result in the chlorine gas migrating to theanolyte headspace via a connection 638 c (FIG. 6C) which optionallyincludes one or more valves or switches 639 to control flow. In at leastsome embodiments, at least a portion of the chlorine gas may be absorbedby the anolyte solution. In at least some embodiments, the followingreactions can occur between chlorine and the anolyte solution tochemically discharge the over-charged system:

2Cr²⁺+Cl₂→2Cr³⁺+2Cl⁻

2Fe²⁺+Cl₂→2Fe³⁺+2Cl⁻

In at least some embodiments, the redox flow battery system 100 mayinclude a pressure release system to manage pressure in the catholyte oranolyte headspace. For example, a pressure relief valve 638 a (FIG. 6A)or a liquid-containing U-tube arrangement 638 b (FIG. 6B) may be coupledto the catholyte headspace to manage the pressure. Similarly, a pressurerelief valve or a liquid-containing U-tube arrangement may be coupled tothe anolyte headspace. In at least some embodiments, gas in the anolyteor catholyte headspace may exchange with an environmental atmosphere viaa bi-directional gas pressure control system such as the U-tubearrangement. In at least some embodiments, a U-tube arrangement may alsobe used as a gas leak monitor. In at least some embodiments, the liquidin a U-tube arrangement may contain an acid level indicator that can beused to estimate the amount of acid-containing gas released into theenvironment by the redox flow battery system.

In at least some instances, the acidic solutions and chemical vapor fromleaks of the electrolytes and chemical products of the redox reactionscan damage electronic devices (for example, the controller 128,switches, valves, pumps, sensors, or the like) in the redox flow batterysystem 100. In addition, the leaks may result in environmental damage orcontamination.

In at least some embodiments, all, or a portion, of the redox flowbattery system 100 that contains the anolyte or catholyte or both can besituated in a secondary container 790 (FIG. 7) that contains acidabsorbent material, such as sodium carbonate, sodium bicarbonate,calcium carbonate, or calcium oxide or the like. In at least someembodiments, the secondary container can contain sufficient acidabsorbent material to neutralize at least 10, 25, 40, 50, 60, 70, 75, 90percent or more of the anolyte or catholyte or both.

In some embodiments, the anolyte and catholyte containing components,such as the anolyte or catholyte tanks 116, 118, half-cells 106, 108, atleast some portions of the anolyte or catholyte distribution systems124, 126, electrodes 102, 104, or the like, of the redox flow batterysystem 100 are maintained at a temperature of at least 50, 60, 70, or 80degrees Celsius or more during charge or discharge periods in atemperature zone 892, as illustrated in FIG. 8. The temperature of thesecomponents may be maintained using one or more heating devices 894. Inaddition, one or more of electronic components of the redox flow batterysystem, such as one or more of the controller 128, the pumps 120, 122,one or more sensors, one or more valves, or the like, are maintained ata temperature of no more than 40, 35, 30, 25, or 20 degrees Celsius orless. The temperature of these components may be maintained using one ormore cooling devices 896.

In at least some embodiments, chromite ore can be used as a startingmaterial to obtain the chromium-containing compound used in the anolyteor catholyte described above. Chromite ore (a mixture of iron andchromium oxides represented by the chemical formula FeO—Cr₂O₃) istreated under high temperature (at least 1000° C.) and under reducingconditions with carbon sources to convert the ore to a porous Fe—Cralloy with a pre-determined content of un-burned carbon particles.

FeCr₂O₄+C→Fe—Cr—C+CO+CO₂

Examples of suitable carbon sources include, but are not limited to,graphite, coal, activated carbon, charcoal, carbon monoxide gas, andcarbon-containing materials containing carbon with oxidation state lessthan 4 which can remove oxygen from the chromite ore as carbon monoxideor carbon dioxide.

The Fe—Cr—C particles are crushed to a predetermined size and thendissolved in hot sulfuric acid in air to generate FeSO₄ and Cr₂(SO₄)₃.Optionally the solution is filtered to remove insoluble components.

Fe+Cr+H₂SO₄→Fe₂(SO₄)₃+Cr₂(SO₄)₃+H₂

Calcium chloride or barium chloride is added to the solution to removemost of the sulfate anions. The solution is filtered to remove calciumsulfate or barium sulfate and other insoluble components.

CaCl₂+Fe₂(SO₄)₃+Cr₂(SO₄)₃→CaSO₄+FeCl₃+CrCl₃

BaCl₂+Fe₂(SO₄)₃+Cr₂(SO₄)₃→BaSO₄+FeCl₃+CrCl₃

Optionally, the Cr:Fe ratio can be adjusted to achieve the final desiredratio (e.g., 3:2 Cr:Fe) by adding FeCl₃ or CrCl₃ to the solution. Thesolution is cooled to crystallize the FeCl₃/CrCl₃ mixture and removeimpurities. Optionally, recrystallization can be used once or multipletimes to remove impurities.

The FeCl₃/CrCl₃ mixture can be used as the electrolyte by adding waterand reduced iron powder to generate a FeCl₂/CrCl₃ solution. Heat fromthe reaction of reduced iron and FeCl₃ can heat the solution if desired.

Fe+2FeCl₃→2FeCl₂+heat (approximately 168.3 kJ/mol of iron)

In at least some embodiments, H₂SO₄ or HCl is added to the solution toproduce the final electrolyte composition.

Other chromium material can also be used. Such chromium materials caninclude chromium waste materials, such as platting wastes, leathertanning wastes and the like (including chromium-containing materialswith Cr⁶⁺ compounds which can be first reduced by reductants such asiron power or Fe²⁺ or Cr³⁺ compounds). These chromium materials can bedissolved using acids, such as hydrochloric acid or sulfuric acid, togenerate chromium salts. The pH of the dissolved chromium can beincreased to pH >3, 5, 7, or more to produce Cr(OH)₃. In at least someembodiments, the choice of acid and pH can provide other chromiumcompounds, such as, for example:

Cr(OH)₃+3HCl→CrCl₃+3H₂O

2Cr(OH)₃+3H₂SO₄→Cr₂(SO₄)₃+6H₂O

Alternatively, adding FeCl₂, Fe+FeCl₃, Fe(OH)₂, or Fe+Fe(OH)₃ incombination with HCl can produce the electrolyte composition.

Chromite ore and chromium waste materials can contain impurities, suchas silica, alumina, iron, and other metals (Ni, Mn, Cu, Sb, Bi, or thelike). Some of these metals can act as hydrogen generation catalysts,promoting hydrogen gas during the Fe—Cr redox flow battery operation. Asdescribed above, the metal impurities can be reduced to metal particlesin the anolyte and a filter, such as filter 121 of FIG. 12, can be usedto remove these particulates. In at least some embodiments, the filter121 has a pore size in a range of 1-10 micrometers. The filter isperiodically cleaned with an oxidizing solution, such as Fe³⁺-containingsolution or the catholyte solution of the flow battery, to dissolvethese small metal particles captured by the filter, and the used cleansolution is removed out of the system. The captured and removed metalcations can be recycled for further applications.

In at least some embodiments, the redox battery system 100 can include asecondary redox flow battery arrangement 201 utilizing the same anolyte112 and catholyte 114, as illustrated in FIG. 10. The secondary redoxflow battery arrangement 201 includes two electrodes 202, 204,associated half-cells 206, 208, a separator 210, an anolyte pump 220, acatholyte pump 222, an anolyte distribution arrangement 224, and acatholyte distribution arrangement 226. The secondary redox flow batteryarrangement 201 also includes the anolyte tank 116 and catholyte 118used by the primary redox flow battery arrangement 101 and is operatedby the same controller 128 and coupled to or coupleable to the sameload/source 130/132, as illustrated in FIG. 10. (Although FIG. 10 mightappear to illustrate the anolyte and catholyte tanks 116, 118 splitbetween the primary and secondary redox flow battery arrangements 101,201, it will be understood that the entirety of the anolyte andcatholyte tanks 116, 118 are part of each of the primary and secondaryredox flow battery arrangements 101, 201.)

In at least some embodiments, the secondary redox flow batteryarrangement 201 has smaller half-cells 206, 208 or anolyte/catholytepumps 220, 222 with more limited pumping capacity (or any combination ofthese features) than the primary redox flow battery arrangement 101. Inat least some embodiments, the ratio of peak power delivery capacity ofthe secondary redox flow battery arrangement 201 to the peak powerdelivery capacity of the primary redox flow battery arrangement 101 isin a range of 1:5 to 1:200 or in a range of 1:1.1 to 1:10 or in a ratioof 1:1.5 to 10. In at least some embodiments, a peak power deliverycapacity of the secondary redox flow battery arrangement is less than apeak power delivery capacity of the primary redox flow batteryarrangement.

In at least some embodiments, the secondary redox flow batteryarrangement 201 can be used to supplement the primary redox flow batteryarrangement 101 during periods of high-power delivery by placing thesecond redox flow battery arrangement 201 in a series or parallelconfiguration with respect to the load 130.

In at least some embodiments, the secondary redox flow batteryarrangement 201 can be used to replace the primary redox flow batteryarrangement 101 during periods of low power delivery to provide periodsof time when the primary redox flow battery arrangement 101 is notoperational or is in a quiescent state. The secondary redox flow batteryarrangement 201 continues to use the same anolyte/catholyte 112, 114 todischarge these electrolytes and reduce or prevent self-discharge oroverheating during periods of nonoperation of the primary redox flowbattery arrangement 101. In at least some embodiments, the secondaryredox flow battery arrangement 201 can be used to restart the primaryredox flow battery arrangement 101.

In at least some embodiments, the controller 128 is configured to switchfrom the primary redox flow battery arrangement 101 to the secondaryredox flow battery arrangement 201 when the discharge power being drawnis no more than a first predefined level. In at least some embodiments,the controller 128 is configured to switch from the secondary redox flowbattery arrangement 201 to the first redox flow battery arrangement 101when the discharge power being drawn is at least a second predefinedlevel. In at least some embodiments, the first and second predefinedlevel are the same. In other embodiments, the second predefined level isgreater than the first predefined level. In at least some embodiments,the controller 128 is configured to add the secondary redox flow batteryarrangement 201 to the first redox flow battery arrangement 101 when thedischarge power being drawn is at least a third predefined level.

In at least some embodiments, the secondary redox flow batteryarrangement 201 can be used as a stand-by power source. In at least someembodiments, when the primary redox flow battery arrangement 101 is tobe placed in a quiescent or non-operational state, the electrolyte 112,114 in the half-cells 106, 108 of the primary redox flow batteryarrangement can be discharged (for example, to avoid damage to theprimary redox flow battery arrangement) by coupling the secondary redoxflow battery arrangement 201 to the primary redox flow batteryarrangement as a load 130. In at least some embodiments, the anolyte andcatholyte pumps 120, 122 are halted or pumping relatively slowly duringthis process because the objective is simply to discharge theelectrolyte 112, 114 in the half-cells 106, 108. Conversely, when thesecondary redox flow battery arrangement 201 is to be placed in aquiescent or non-operational state, the electrolyte 112, 114 in thehalf-cells 206, 208 of the secondary redox flow battery arrangement canbe discharged by coupling the primary redox flow battery arrangement 201to the secondary redox flow battery arrangement as a load 130.

It will be recognized that a redox flow battery system can include oneor more primary redox flow battery arrangements 202 and one or moresecondary redox flow battery arrangements 201. It will be recognizedthat one or more second redox flow battery arrangements 201 can beincorporated in any of the other embodiments described herein and thatany of the modifications described herein for a redox flow batterysystem can also be applied to the secondary redox flow batteryarrangement 201.

The redox flow battery can be used for the production of hydrogen byelectrolysis of water 1112 in an electrolysis cell 1130 attached to theredox cell battery system 100, as illustrated in FIG. 11. Theelectrolysis cell 1130 includes electrodes 1102, 1108 and a hydrogenremoval arrangement 1125 delivers the hydrogen gas 1113 generated byelectrolysis of water in the electrolysis cell 1130 to a hydrogen gasstorage tank 1117. Separation of the hydrogen gas 1113 from oxygen gasis known and can be performed using any suitable method.

The redox flow battery system 100 provides a convenient and usefulmechanism for generating hydrogen gas. The redox flow battery system 100can be charged and then used to electrolyze water in the electrolysiscell 1130. In contrast to many conventional arrangements for hydrogenproduct that require converting AC power to DC power to hydrolyze water,the redox flow battery system 100 inherently generates DC power whichcan be sufficient to hydrolyze the water to generate hydrogen gas.

Alternatively or additionally, in at least some embodiments, hydrogengas 1113 is a byproduct of operation of the redox flow battery system100. A hydrogen removal arrangement (similar to hydrogen removalarrangement 1125) delivers the hydrogen gas 1113 generated in either thefirst or second half cells 106, 108 to the hydrogen gas storage tank1117. This byproduct of the charging of the redox flow battery system100 can be sold as an additional product.

In a redox flow battery system, the Average Oxidation State (AOS) of theactive species in the catholyte or the anolyte (or both) can change,particularly under conditions such as hydrogen generation or oxygenintrusion in the system. As a result of the AOS change, the system canbecome unbalanced, the system storage capacity may decrease, or sidereactions, such as hydrogen generation, may be accelerated, or anycombination of these effects.

It is useful to know the AOS of a redox flow battery system.Conventional AOS determination methods include taking samples andconducting an off-line potential titration analysis, obtaining in situUV-Vis measurement; or performing in situ potential differencemeasurements against a reference electrode. These techniques can be slowor may be relatively inaccurate.

In contrast to these conventional techniques, relatively fast andaccurate methods are presented herein. The method includes measuring thecapacity of ions in at least one of the electrolytes during a lowpotential charging process and using this charging capacity and theknown volume of the electrolyte to determine the AOS.

In at least some embodiments, the anolyte/catholyte tanks 116, 118 caneach be a set of tanks. For example, in at least some embodiments, theanolyte/catholyte tanks 116, 118 each include a primary electrolyte tank2116 and one or more supplemental electrolyte tanks 2118, as illustratedin FIG. 21. In at least some embodiments, the primary electrolyte tank2116 is cylindrical, as illustrated in FIG. 21, as opposed to being acube or rectangular cuboid. In at least some embodiments, a cube orrectangular cuboid is prone to leakage, particularly for relativelylarge tanks, whereas a cylindrical tank is less likely to leak.

In at least some embodiments, a tank storage region 125 for a redox flowbattery system may be a rectangular cuboid, as illustrated in FIG. 21,or another non-cylindrical shape. A cylindrical primary electrolyte tank2116 may not fill the tank storage region 125, as illustrated in FIG.21. To provide additional electrolyte storage, one or more (for example,one, two, three, four, five, six, or more) supplemental electrolytetanks 2118 can be included in the tank storage region 125. Thesupplemental electrolyte tanks 2118 can be connected in series orparallel to the primary electrolyte tank 2116. The supplementalelectrolyte tanks 2118 can be cylinders, cubes, rectangular cuboids, orany other suitable shape. Concerns with leakage (or the likelihood ofleakage) may be lower if the supplemental electrolyte tanks 2118 have asubstantially smaller volume (for example, no more than 5, 10, or 25%)than the primary electrolyte tank 2116. The supplemental electrolytetanks 2118 can all have the same shape or volume or can have differentshapes or volumes.

FIG. 13 illustrates three different charging curves for a redox flowbattery system. The first charging curve 990 is for a balanced system.The second charging curve 992 and third charging curves 994 are for twodifferent un-balanced systems. In the unbalanced systems, there is a lowpotential charging region 992 a, 994 a and a high potential chargingregion 992 b, 994 b. By performing one or more measurements in the lowpotential charging region 992 a, 994 a, the AOS can be determined.

This AOS determination methods will be illustrated using the Fe—Cr redoxflow battery system described above. It will be understood, however,that these methods can be used with any other suitable redox flowbattery system. In this example, both the catholyte and the anolyteinclude iron and chromium ions. For a balanced system, when the redoxflow battery system is fully discharged, there is only Fe²⁺ and Cr³⁺ inboth the anolyte and catholyte. Upon application of a charge, the systemundergoes the following redox reactions:

Fe²⁺⁻→Fe³⁺+e (E^(o)=+0.77 V, positive side)

Cr³⁺+e⁻→Cr²⁺ (E^(o)=−0.40 V, negative side)

The potential difference between the positive and negative electrolytescan increase to more than 0.9 V immediately in at least someembodiments, as illustrated by first charging curve 990 in FIG. 13.

If the system is unbalanced, however, AOS increases due to sidereactions. For example, a mixture of Fe²⁺/Fe³⁺/Cr³⁺ can be found in thedischarged electrolyte of an unbalanced Fe—Cr redox flow battery system.For such a mixture of ions, when a charge is applied to the redox flowbattery system, the system first undergoes the following redoxreactions:

Fe²⁺⁻→Fe³⁺+e (positive side)

Fe³⁺+e⁻→Fe²⁺ (negative side)

These reactions occur at low potential until all of the Fe³⁺ ions areconsumed at the negative electrolyte. This corresponds to the lowpotential charging region 992 a, 994 a in FIG. 13. Only after consumingall of the Fe³⁺ ions will the chromium ions be reduced at the highercharging potential, as illustrated in high potential charging regions992 b, 994 b.

The charge capacity of this low potential charging process can be usedto determine the amount of Fe³⁺ ions in the anolyte. For example, thecharge capacity at the turning point 993 of FIG. 13 divided by 26.8Ah/mole (the charge on one mole of electrons) gives the molar amount ofFe³⁺ in the anolyte. This combined with the volume of the positive andnegative electrolytes can be used to determine the AOS of the redox flowbattery system.

This information can also be used to rebalance the redox flow batterysystem using, for example, the balance arrangements described above. Asan example, using the balance arrangement 500 illustrated in FIGS. 5Aand 5B, a balancing electrolyte 562 containing oxovanadium ions, and areductant 563 containing fructose, along with the application of anexternal potential from the potential source 561 of at least 0.23 Vproduces the following reactions:

VO²⁺+H₂O+Fe³⁺→VO₂ ⁺+Fe²⁺+2H⁺

24VO₂ ⁺+24H⁺+C₆H₁₂O₆→24VO²⁺+6CO₂+18H₂O

Rebalancing the AOS can be accomplished by providing the reductant 563(fructose) to the balancing electrolyte 562. For example, ideallyproviding a molar amount of fructose equal to the molar amount of Fe³⁺multiplied by ( 1/24) can rebalance the AOS. It will be recognized thatmore fructose may be used to fully rebalance the AOS due to thenon-ideal elements in the system.

FIG. 14 is a flowchart of one embodiment of a method of determining AOS.In step 1060, the charge capacity during a low potential charging periodis measured. For example, the charge capacity can be measured as thecharge capacity when the charging curve changes from the low potentialcharging period to the high potential charging period, such as, forexample, at turning points 993, 995 in FIG. 13. In step 1062, the AOScan then be determined using this measurement (or set of measurements)and the known volumes of the anolyte and catholyte and the known ironand chromium concentrations of the catholyte and anolyte, respectively.In at least some embodiments, prior to step 1060, the anolyte 112 andcatholyte 114 in the redox flow battery system 100 can be fullydischarged by applying an external potential or by complete mixing ofthe anolyte and catholyte. In at least some embodiments, the catholyteand anolyte may be fully mixed prior to determining the charge capacity.

Another embodiment does not require a fully discharged system, but canbe determined for any state-of-charge condition using columbictitration. FIG. 15 is a flowchart of one embodiment of this method. Instep 1166, the amount of one ionic species (for example, Fe³⁺ or Cr³⁺)is measured in one of the electrolytes (catholyte or anolyte) by haltingthe flow of that electrolyte and performing in situ titration of thefirst ionic species in the non-flowing electrolyte. The balancearrangement 500 or the other electrolyte, which is preferably continuingto flow through the redox flow battery system, can be used to titratethe ionic species. As an example, the amount of Fe³⁺ can be determinedin the catholyte by halting flow of the catholyte and titrating the Fe³⁺using either a) the vanadium ions in the balance arrangement 500 or b)the Cr²⁺ in the anolyte, which may continue to flow through the redoxbattery system in order to ensure that there is sufficient Cr²⁺ ions totitrate all of the Fe³⁺.

In step 1168, the amount of a second ionic species (for example, Cr²⁺ orFe²⁺) is measured in one of the electrolytes (anolyte or catholyte) byhalting the flow of that electrolyte and performing in situ titration ofthe second ionic species in the non-flowing electrolyte. Preferably, thesecond ionic species is measured in a different electrolyte from theelectrolyte in which the first ionic species is measured so that oneionic species is measured in the catholyte and one ionic species ismeasured in the anolyte. In at least some embodiments, the measurementof the first ionic species will alter the amount or concentration of thesecond ionic species so that the measurement of the second ionic specieswill be adjusted to take into account this alteration. The balancearrangement 500 or the other electrolyte, which is preferably continuingto flow through the redox flow battery system, can be used to titratethe ionic species. As an example, the amount of Cr²⁺ can be determinedin the anolyte by halting flow of the anolyte and titrating the Cr²⁺using either a) the vanadium ions in the balance arrangement 500 or b)the Fe³⁺ in the catholyte, which may continue to flow through the redoxbattery system in order to ensure that there is sufficient Fe³⁺ ions totitrate all of the Cr²⁺.

In step 1170, the AOS can then be determined using these twomeasurements, the initial concentrations or amounts of iron andchromium, the volumes of the half-cells, and the volumes of the anolyteand catholyte.

In another embodiment, the discharge process, instead of the chargingprocess, can be observed. For a charged or partly charged system, adischarge or self-discharge process can be used to estimate the extraFe³⁺ in the positive electrolyte based on the discharge rate differencebetween two different electrochemical pairs (here, Fe²⁺/Fe³⁺ vsCr²⁺/Cr³⁺) and one pair with different concentrations (Fe²⁺/Fe³⁺). Thechange in discharge rate for the two electrochemical pairs in anelectrochemical device is much faster than that of one electrochemicalpair with different concentrations. As a result, as illustrated in FIG.16 a turning point 1297 can be observed in the discharge curve 1296. Inat least some embodiments, this turning point 1297 can be used toestimate the amount of extra Fe³⁺ in the catholyte. FIG. 17 is oneembodiment of a method of determining the AOS using the discharge curve.In step 1374, the discharge rate is measured during initial discharge.In step 1376, a turning point in the voltage discharge is determined andan open circuit voltage is measured. In step 1378, the open circuitvoltage after the turning point can be used to estimate the extraconcentration of the active electrochemical pair (here, Fe³⁺/Fe²⁺⁺)using the Nernst equation. In at least some embodiments, the dischargeend point is selected to be the point at which there is no more than 1%,5%, or 10% Cr²⁺ ions (of the total chromium) in the anolyte. In step1380, the AOS can be determined from the estimated concentration, andknown volumes of the electrolytes. This is the reverse process of thepreviously described method illustrated in FIG. 14.

In at least some embodiments, the methods of determining AOS describedabove and illustrated using FIGS. 13 to 18 can be performed in situusing the half-cells 106, 108, anolyte 112, catholyte 114, otherelements of the redox flow battery system 100, or elements of thebalance system 500. In other embodiments, the methods of determining AOSmay include flowing a portion of the anolyte 112 or catholyte 114 orboth into one or more other half-cells for measurements. In yet otherembodiments, the methods of determining AOS can include removingportions of the anolyte 112 or catholyte 114 or both and performingmeasurements external to the redox flow battery system 100.

In at least some embodiments, the determined AOS can be used to estimatethe amount of hydrogen generated or the production of other sideproducts. In at least some embodiments, the determination of AOS inFIGS. 14, 15, and 17 can be followed by operation of a balancingarrangement, as described above, to rebalance the redox flow batterysystem and restore the AOS. Such operation can include, for example,determining an amount of the reductant 563 to add to the balancingelectrolyte 562.

For a given redox flow battery system, there is a fixed ratio of theelectrolyte volume inside the battery stack and the whole batterysystem. The electrolyte volume inside the battery stack is always muchsmaller than that outside the electrolyte tanks. Thus, it is muchquicker to charge and discharge the electrolyte inside the stack than tocharge and discharge the electrolyte in the whole system. The solutionfor a quick measurement for the available capacity of the redox flowbattery is the charge or discharge of the electrolyte in the stack onlywithout electrolyte flow at a given operation condition and then convertthe result to the whole system based on the system design parameters.

There is often a desire to know the available energy or storagecapacity, or a change in that energy or storage capacity, of a redoxflow battery system. Conventional methods for such determinationinclude, for example, taking samples from the redox flow battery systemto conduct off-line titration or other analysis, in situultraviolet-visible (UV-Vis) measurements, or in situ potentialdifference measurements against a reference electrode. These methods canbe slow or inaccurate.

It has been found that, during the operation of a redox flow batterysystem, when the active material become unbalanced, the end OCV (opencircuit voltage) of the redox flow battery system for a given set ofdischarge conditions changes. The difference in the end OCV has a directrelationship with its usable storage or charge capacity. In addition,the end OCV can have a direct relationship with the AOS. In particular,for a given redox flow battery system, under the same discharge rate,there is a reliable relationship between the end OCV and the systemstorage or charge capacity or AOS. Under certain conditions, such as H₂generation in the anolyte side of the system or O₂ intrusion into thesystem, the active species in the system become unbalanced. As a result,the system storage or charge capacity decreases and side reactions arefurther accelerated. The relationship between the OCV and the systemstorage or charge capacity or AOS also changes.

As an example, for one embodiment of a Fe—Cr redox flow battery system,after being charged to an OCV of 1.100V and this discharged at 33 mW/cm²to 0.60V, the discharge energy and related end OCV are given in thetable below.

End OCV (V) 0.874 0.872 0.865 0.860 0.854 0.850 0.848 Discharge 890 900926 949 968 986 1001 Energy (Wh)As shown in this table, the end OCV under the same discharge conditionscan be used as an indicator for the available storage or charge capacityof the system. Accordingly, the end OCV can also be used as an indicatorof the AOS.

Thus, in at least some embodiments, the available storage or chargecapacity or the AOS can be determined by total discharge of the systemunder a set of given conditions, such as at a preselected discharge rateor power, followed by measurement of the end OCV. This end OCV can becompared to a pre-determined OCV curve; a pre-determined look-up tableor other calibration table, chart, or the like; or applied to apre-determined mathematical relationship to determine the storage orcharge capacity or the AOS of the redox flow battery system.

In at least some embodiments, the discharge and end OCV measurement canbe performed using the half-cells 106, 108 and electrodes 102, 104. Inat least some embodiments, the entire redox flow battery system isdischarged.

For a redox flow battery system, there is typically a fixed ratio of theelectrolyte volume in the battery stack (e.g., half-cells 106, 108) andthe whole system. The electrolyte volume inside the battery stack istypically much smaller than the volume in the electrolyte tanks 116,118. Thus, it can be much quicker to charge and discharge theelectrolyte inside the half-cells 106, 108 than to charge and dischargethe electrolyte in the whole system. Accordingly, in at least someembodiments, flow of the electrolytes (anolyte and catholyte) may behalted during this storage capacity determination so that only theelectrolyte in the half-cells 106, 108 is discharged. Typically, the endOCV measurement of the electrolytes in the half-cells 106, 108 isindicative of the entire redox flow battery system as a whole.

Moreover, it may be advantageous to use a cell with even smaller volumethan the half-cells of the battery stack (for example, half-cells 106,108) to perform the discharge and measurement of end OCV. In at leastsome embodiments, the redox flow battery system 100 can include an OCVcell 1401 with half-cells 1406, 1408 and electrodes 1402, 1404, asillustrated in FIG. 18. In at least some embodiments, the OCV cell 1401may be at least 25%, 30%, 40%, 50%, 60%, or 75% smaller in volume thanthe half-cells 106, 108 and electrodes 102, 104. The OCV cell 1401 maybe in-line with the flow of electrolytes (anolyte and catholyte) throughthe half-cells 106, 108, as illustrated in FIG. 18. Alternatively, theOCV cell 1401 may be positioned outside of the in-line flow of theelectrolytes through the anolyte distribution arrangement 124 andcatholyte distribution arrangement 126 and the OCV cell 1401 can befilled with the electrolytes using valves, switches, or the like. It maybe advantageous to have a relatively small OCV cell 1401 as thedischarge process in the OCV cell may be faster than in the half-cells106, 108 (with the flow of electrolytes halted) resulting in a fastermeasurement of the end OCV and determination of the storage capacity.Accordingly, in at least some embodiments, flow of the electrolytes(anolyte and catholyte) may be halted during this storage capacitydetermination so that only the electrolyte in OCV cell 1401 isdischarged.

FIG. 19 is a flowchart of one embodiment of a method of determiningstorage or charge capacity or AOS of a redox flow battery system. Instep 1574, the redox flow battery system is discharged at a preselecteddischarge rate. In at least some embodiments, the actual discharge ratevaries from the predetermined discharge rate by no more than 1, 5, or10%. In at least some embodiments, the discharge is a self-discharge ofthe redox flow battery system. In at least some embodiments, the enddischarge point is when the amount of Cr²⁺ ions in the anolyte is nomore than 1%, 5%, or 10% of the total chromium. Other end dischargepoints can be used.

In step 1576, after the discharge, the end OCV is measured. In step1578, the measured end OCV is compared to a pre-determined OCV curve; apre-determined look-up table or other calibration table, chart, or thelike; or applied to a pre-determined mathematical relationship thatrelates the end OCV to concentrations of one or more active species, thestorage or charge capacity, or the AOS of the redox flow battery system.In at least some embodiments, a pre-determined end OCV curve; apre-determined look-up table or other calibration table, chart, or thelike; or applied to a pre-determined mathematical relationship isobtained by experimentally measuring the end OCV, using the preselecteddischarge rate, of redox flow battery systems with different values ofconcentrations of one or more active species, storage, or chargecapacity, or AOS.

In optional step 1580, when the concentrations of one or more activespecies or the storage or charge capacity is determined in step 1578,the concentrations of one or more active species or storage or chargecapacity can be used to determine the AOS using the electrolyte volumes.

In at least some embodiments, when the storage or charge capacity or AOSis determined and indicates that the system is unbalanced, any of thetechniques described above can be employed to rebalance the redox flowbattery system.

AOS can also be determined using other measurements of OCV. FIG. 20 is aflowchart of one embodiment of a method of determining AOS of a redoxflow battery system. In step 1680, the OCV of a redox flow batterysystem is measured. In step 1682, one or both of the followingprocedures are performed: a) an amount of one iron ionic species (forexample, Fe³⁺ or Fe²⁺) in the catholyte is measured; or b) an amount ofone chromium ionic species (for example, Cr³⁺ or Cr²⁺) in the anolyte ismeasured. It will be recognized that measurement of other ionic speciesin either the anolyte or catholyte (or in other types of redox flowbattery systems) can be used in this step. In at least some embodiments,the two half-cells used for measurement of the amount of the one ironionic species or the one chromium ionic species can be part of thebattery stack of the redox flow battery system, such as half-cells 106,108. In other embodiments, the two half-cells are not part of thebattery stack, but can be, for example, the OCV cell 1401 of FIG. 18. Inat least some embodiments, the measurement is made without electrolyteflow in the redox flow battery system. In at least some embodiments, theflow of the electrolyte that is the object of the measurement is haltedwhile the flow of the other electrolyte is maintained to facilitatecomplete titration of the ionic species being measured. In at least someembodiments, the measurement of the one iron ionic species or the onechromium ionic species can utilize the balance arrangement 500 of FIG.5A and the measurement steps can include the reduction of the one ironionic species or the one chromium ionic species and the oxidation ofvanadium ions followed by the regeneration of the vanadium ions byreducing the dioxovanadium ions using a reductant, as described above.In at least some embodiments, the measurement of the one iron ionicspecies or the one chromium ionic species can be performed off-line orcan be performed in situ.

In step 1684, the AOS is determined using i) the measured amount of theone iron ionic species, the measured amount of the one chromium ionicspecies, or the measured amounts of both of the one iron ionic speciesand the one chromium ionic species, ii) the measured OCV, and iii) arelationship between the amount or concentration of the one iron ionicspecies or the one chromium ionic species in the catholyte or anolyte,respectively, and the OCV. In at least some embodiments, the AOS can bedetermined from a pre-determined OCV curve, look-up table, calibrationtable, or mathematical relationship relating OCV to one of thefollowing: a concentration or amount of the one iron ionic species, aconcentration or amount of the one chromium ionic species, orconcentrations or amounts of both the one iron ionic species or the onechromium ionic species.

In at least some embodiments, the pre-determined OCV curve, look-uptable, calibration table, or mathematical relationship can be arelationship between the OCV and the measured or calculated ionicspecies for a balanced system. The measured OCV can provide an expectedconcentration or amount of that ionic species. The difference between 1)the expected concentration or amount of the measured ionic speciesversus 2) the measured concentration or amount of the ionic species canbe used to determine the AOS.

In at least some embodiments, the pre-determined OCV curve; apre-determined look-up table or other calibration table, chart, or thelike; or applied to a pre-determined mathematical relationship isobtained by experimentally measuring the OCV for different amounts orconcentrations of the selected ionic species in a balanced system.

In at least some embodiments, when the AOS is determined and indicatesthat the system is unbalanced, any of the techniques described above canbe employed to rebalance the redox flow battery system.

The charging and discharging of the anolyte/catholyte can depend on avariety of factors including the pumping rate, the power delivered by acharging source, or the power required by a load. In at least someinstances, the charging/discharging of the anolyte/catholyte in thehalf-cells is not constant but varies over time. This can result intemporal variation of the AOS (average oxidation state) or SOC (state ofcharge) of the anolyte/catholyte entering or exiting the half-cells,particularly when there is limited or no mixing of theanolyte/catholyte. (In at least some instances, mixing theanolyte/catholyte to obtain relatively uniform AOS or SOC throughout theanolyte/catholyte may be challenging.)

In at least some embodiments, the AOS or SOC (or other value indicativeof the AOS or SOC) of the anolyte 112 or catholyte 114 (or both) can beintermittently, periodically, or continuously measured or estimated whenthe anolyte or catholyte enters or exits the half-cells 106, 108 (orelsewhere along the anolyte or catholyte distribution arrangements 124,126) to record a temporal energy profile of the anolyte 112 or catholyte114. The temporal energy profile can represent the AOS or SOC ofdifferent portions of the anolyte/catholyte and can correspond to thevariation profile in the AOS or SOC through the entire volume of theanolyte/catholyte.

In at least some embodiments, the temporal energy profile may correspondto measurement, calculations, or estimations of the AOS or SOC or of aquantity that is indicative of the AOS or SOC, such as a measuredvoltage of the anolyte or catholyte. In at least some embodiments, theAOS or SOC is determined or estimated for the temporal energy profile.In at least some embodiments, the temporal energy profile includes (oris based on) intermittent, periodic, or continuous measurements or othercalculated values that are indicative of (for example, proportional toor relative in a linear or non-linear manner to) the AOS or SOC.

In at least some embodiments, the open circuit voltage (or othermeasurement indicative of the AOS or SOC) of the anolyte 112 orcatholyte 114 (or both), as well as the charge/discharge current, areintermittently, periodically, or continuously measured or estimated asthe anolyte or catholyte pass through the half-cells. In at least someembodiments, the AOS or SOC can be determined using the open circuitvoltage and the charge/discharge current and this can be used togenerate the temporal energy profile. In at least some embodiments, asmall open circuit voltage measurement cell can be used to measure theopen circuit voltage (OCV) of the anolyte and catholyte before or afterpassing the through the half-cells using, for example, a small portionof anolyte and catholyte diverted to the open circuit voltagemeasurement cell.

In at least some embodiments, the redox flow battery system 100 ismaintained to reduce mixing or diffusion of the anolyte/catholyte sothat the temporal energy profile of the anolyte 112 or catholyte 114 isreliable along the anolyte/catholyte distribution arrangement 124, 126and within the volume of the anolyte/catholyte. In at least someembodiments, the temporal energy profile may be modified to account fordiffusion within the anolyte/catholyte using an estimated diffusioncoefficient and one or more factors such as, for example, a) the timefor the anolyte/catholyte to travel through the anolyte/catholytedistribution arrangement after exiting the half-cell until reenteringthe half-cell, b) the AOS or SOC of temporally adjacent portions of theanolyte/catholyte, c) the temperature of the anolyte/catholyte, theelectrolyte flow rates, or the like or any combination thereof.

In at least some embodiments, the temporal energy profile can be used tovary the speed of the anolyte/catholyte pumps 120, 122 to providesufficient power for a load 130. For example, the pumping speed of theanolyte/catholyte pumps 120, 122 can be increased when the temporalenergy profile indicates that the anolyte/catholyte 112, 114 enteringthe half-cells 106, 108 has lower charge as indicated by the measured orestimated AOS or SOC. The pumping speed can also be varied based on theamount of power being drawn by the load 130.

FIG. 22 illustrates a redox flow battery arrangement that includes oneor more state measurement devices 123 for making a measurement tofacilitate measurement, determination, or estimation of the AOS or SOC(or of a quantity that is indicative of the AOS or SOC) of the anolyte112 or catholyte 114 to produce a temporal energy profile of the anolyteor catholyte. For example, the state measurement device 123 can measurea voltage of the anolyte or catholyte, concentrations of active species(e.g., Cr³⁺, Cr²⁺, Fe³⁺, or Fe²⁺) by a spectrometer, or the like of theanolyte 112 or catholyte 114. FIG. 22 illustrates examples of positionsof the state measurement device(s) 123, but it will be understood thatsuch state measurement device(s) can be positioned anywhere along thepath of flow of the anolyte 112 or catholyte 114 including, but notlimited to, before entering or after leaving the half-cells 106, 108.

In at least some embodiments, a state measurement device 123 ispositioned for making a measurement of only the anolyte 112 or thecatholyte 114. Such an arrangement may work best if the balance of theanolyte/catholyte is maintained.

In at least some embodiments, the temporal energy profile can be atleast partially determined or estimated using a temporal chargingprofile obtained by intermittent, periodic, or continuous measurement ofthe power supplied by the charging source 132 using a chargingmeasurement device 127, as illustrated in FIG. 22. In at last someembodiments, the charging profile can be combined with thecontemporaneous pumping speed of the anolyte/catholyte pump 120, 122 toobtain the temporal energy profile of the anolyte/catholyte 112, 114. Inat least some embodiments, the charge or discharge currents arecontinuously recorded during the redox flow battery operation. This canbe combined with, for example, the actual pumping rate and the measuredOCV (for example, the OCV of anolyte/catholyte entering or exiting thehalf-cell), to generate the temporal charging profile.

In at least some embodiments, the controller 128 receives themeasurements from the state measurement device(s) 123 or chargingmeasurement device(s) 127. In at least some embodiments, the controller128 determines, calculates, records, or stores the temporal energyprofile or the temporal charging profile or any combination thereof. Inat least some embodiments, the controller 128 varies the pumping speedof the anolyte or catholyte pumps 120, 122 based on the temporal energyprofile. The pumping rate can also be intermittently, periodically, orcontinuously recorded. In at least some embodiments, the pumping ratecan be determined using the pump's power consumption or actual flow ratemeasurements.

The controller 128 can include at least one process 129 and at least onememory 131. The controller 128 can utilize any suitable processor 129including one or more hardware processors that may be local to the useror non-local to the user or other components of the computer. Theprocessor 129 is configured to execute instructions provided to theprocessor, as described below.

Any suitable memory 131 can be used for the controller 128. The memory131 illustrates a type of computer-readable media, namelycomputer-readable storage media. Computer-readable storage media mayinclude, but is not limited to, nonvolatile, non-transitory, removable,and non-removable media implemented in any method or technology forstorage of information, such as computer readable instructions, datastructures, program modules, or other data. Examples ofcomputer-readable storage media include RAM, ROM, EEPROM, flash memory,or other memory technology, CD-ROM, digital versatile disks (“DVD”) orother optical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, cloud storage, or any othermedium which can be used to store the desired information and which canbe accessed by a processor.

FIG. 23 illustrates one embodiment of operating a redox flow batterysystem, such as the redox flow battery system 100 of FIG. 22. In step2302, the state measurement device(s) 123 is used to intermittently,periodically, or continuously make a measurement of a value indicativeof a state of charge of the anolyte 112 or the catholyte 114 beforeentering or after leaving the half-cell 106 or second half-cell 108,respectively. For example, the state measurement device(s) 123 makes ameasurement of the open circuit voltage of the anolyte or catholyte.

Alternatively or additionally, the charging measurement device(s) 127 isused to intermittently, periodically, or continuously make a measurementof power supplied by a charging source 132.

In step 2304, a temporal profile of the anolyte or the catholyte isgenerated using the measurements. In at least some embodiments, the AOSor SOC of the anolyte or catholyte is determined from the measurement(s)made by the at least one state measurement device and used to generatethe temporal energy profile. In at least some embodiments, an estimateof diffusion of charged species within the anolyte or catholyte is alsoused to generate the temporal energy profile.

In step 2306, during discharging a controller 128 of the system variesthe pump speed of the anolyte/catholyte pumps 120, 122 based on thetemporal energy profile to provide the power required for a load.

It will be recognized that the redox flow battery system of FIG. 22 andthe method of operation of the redox flow battery system of FIG. 23 arenot limited to Fe—Cr redox flow batteries, but can be used in otherredox flow battery system including, but not limited to, vanadium redoxflow battery systems, vanadium-bromine redox flow battery systems,vanadium-iron redox flow battery systems, zinc-bromine redox flowbattery systems, all iron redox flow battery systems, organic aqueousredox flow battery systems, or the like.

The methods, systems, and devices described herein may be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Accordingly, the methods, systems, anddevices described herein may take the form of an entirely hardwareembodiment, an entirely software embodiment or an embodiment combiningsoftware and hardware aspects. The following detailed description is,therefore, not to be taken in a limiting sense. The methods describedherein can be performed using any type of processor and any suitabletype of device that includes a processor.

It will be understood that each block of the flowchart illustrations,and combinations of blocks in the flowchart illustrations and methodsdisclosed herein, can be implemented by computer program instructions.These program instructions may be provided to a processor to produce amachine, such that the instructions, which execute on the processor,create means for implementing the actions specified in the flowchartblock or blocks disclosed herein. The computer program instructions maybe executed by a processor to cause a series of operational steps to beperformed by the processor to produce a computer implemented process.The computer program instructions may also cause at least some of theoperational steps to be performed in parallel. Moreover, some of thesteps may also be performed across more than one processor, such asmight arise in a multi-processor computer system. In addition, one ormore processes may also be performed concurrently with other processes,or even in a different sequence than illustrated without departing fromthe scope or spirit of the invention.

The computer program instructions can be stored on any suitablecomputer-readable medium including, but not limited to, RAM, ROM,EEPROM, flash memory or other memory technology, CD-ROM, digitalversatile disks (“DVD”) or other optical storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices,or any other medium (which may be local or nonlocal to the computer)which can be used to store the desired information and which can beaccessed by a processor.

The above specification provides a description of the manufacture anduse of the invention. Since many embodiments of the invention can bemade without departing from the spirit and scope of the invention, theinvention also resides in the claims hereinafter appended.

What is claimed as new and desired to be protected is:
 1. A redox flowbattery system, comprising an anolyte; a catholyte; a first half-cellcomprising a first electrode in contact with the anolyte; a secondhalf-cell comprising a second electrode in contact with the catholyte; aseparator separating the anolyte in the first half-cell from thecatholyte in the second half-cell; at least one state measurement deviceconfigured for intermittently, periodically, or continuously making ameasurement of a value indicative of a state of charge of the anolyte orthe catholyte before entering or after leaving the first half-cell orsecond half-cell, respectively; and a controller coupled to the at leastone state measurement device for generating a temporal energy profile ofthe anolyte or the catholyte, respectively, using the measurements. 2.The redox flow battery system of claim 1, wherein the at least one statemeasurement device comprises an anolyte state measurement deviceconfigured for intermittently, periodically, or continuously making ameasurement of a value indicative of a state of charge of the anolytebefore entering or after leaving the first half-cell and a catholytestate measurement device configured for intermittently, periodically, orcontinuously making a measurement of a value indicative of a state ofcharge of the catholyte before entering or after leaving the secondhalf-cell.
 3. The redox flow battery system of claim 1, furthercomprising an anolyte pump configured for pumping anolyte into and outof the first half-cell and a catholyte pump configured for pumpingcatholyte into or out of the second half-cell.
 4. The redox flow batterysystem of claim 3, wherein the controller is configured to use thetemporal energy profile to vary pumping speed of the anolyte andcatholyte pumps.
 5. The redox flow battery system of claim 1, whereinthe at least one state measurement device is configured to measure avoltage of the anolyte or catholyte.
 6. The redox flow battery system ofclaim 1, wherein the controller is configured to determine a state ofcharge of the anolyte or catholyte from the measurement made by the atleast one state measurement device and to use the state of charge in thetemporal energy profile.
 7. The redox flow battery system of claim 1,wherein the controller is configured to determine an average oxidationstate of the anolyte or catholyte from the measurement made by the atleast one state measurement device and to use the average oxidationstate in the temporal energy profile.
 8. The redox flow battery systemof claim 1, wherein the controller is configured to generate thetemporal energy profile using the measurements and an estimate ofdiffusion of charged species within the anolyte or catholyte.
 9. Amethod of operating the redox flow battery system of claim 1, the methodcomprising: intermittently, periodically, or continuously making ameasurement of a value indicative of a state of charge of the anolyte orthe catholyte before entering or after leaving the first half-cell orsecond half-cell, respectively; and generating a temporal energy profileof the anolyte or the catholyte, respectively, using the measurements.10. The method of claim 9, wherein the redox flow battery system furthercomprises an anolyte pump configured for pumping anolyte into and out ofthe first half-cell and a catholyte pump configured for pumpingcatholyte into or out of the second half-cell; the method furthercomprising using the temporal energy profile to vary pumping speed ofthe anolyte and catholyte pumps.
 11. The method of claim 9, whereinmaking a measurement comprising making a measurement of a voltage of theanolyte or catholyte.
 12. The method of claim 9, wherein generating thetemporal energy profile comprises determining a state of charge of theanolyte or catholyte from the measurement made by the at least one statemeasurement device and using the state of charge in the temporal energyprofile.
 13. The method of claim 9, wherein generating the temporalenergy profile comprises determining an average oxidation state of theanolyte or catholyte from the measurement made by the at least one statemeasurement device and using the average oxidation state in the temporalenergy profile.
 14. The method of claim 9, wherein generating thetemporal energy profile comprises generating the temporal energy profileusing the measurements and an estimate of diffusion of charged specieswithin the anolyte or catholyte.
 15. A non-transitory computer-readablemedium having processor-executable instructions for operating the redoxflow battery system of claim 1, the processor-executable instructionswhen installed onto a device enable the device to perform actions, theactions comprising: intermittently, periodically, or continuously makinga measurement of a value indicative of a state of charge of the anolyteor the catholyte before entering or after leaving the first half-cell orsecond half-cell, respectively; and generating a temporal energy profileof the anolyte or the catholyte, respectively, using the measurements.16. The non-transitory computer-readable medium of claim 15, the actionsfurther comprise using the temporal energy profile to vary pumping speedof anolyte and catholyte pumps of the redox flow battery system.
 17. Thenon-transitory computer-readable medium of claim 15, wherein making ameasurement comprising making a measurement of a voltage of the anolyteor catholyte.
 18. The non-transitory computer-readable medium of claim15, wherein generating the temporal energy profile comprises determininga state of charge or average oxidation state of the anolyte or catholytefrom the measurement made by the at least one state measurement deviceand using the state of charge or average oxidation state in the temporalenergy profile.
 19. The non-transitory computer-readable medium of claim15, wherein generating the temporal energy profile comprises generatingthe temporal energy profile using the measurements and an estimate ofdiffusion of charged species within the anolyte or catholyte.
 20. Aredox flow battery system, comprising an anolyte; a catholyte; a firsthalf-cell comprising a first electrode in contact with the anolyte; asecond half-cell comprising a second electrode in contact with thecatholyte; a separator separating the anolyte in the first half-cellfrom the catholyte in the second half-cell; at least one chargingmeasurement device configured for intermittently, periodically, orcontinuously making a measurement of power supplied by a chargingsource; and a controller coupled to the at least one chargingmeasurement device for generating a temporal energy profile of theanolyte or the catholyte, respectively, using the measurements.